Valve seat insert of two layers of same compact density

ABSTRACT

A two layer valve seat insert and a method for its manufacture is described. The method comprises the steps of preparing two powder mixtures; a first powder mixture for forming the valve seat face layer; a second powder mixture for forming the valve seat base layer; sequentially introducing a predetermined quantity of each of said first and said second powder mixtures into a powder compacting die and having an interface therebetween substantially perpendicular to the axis of said die; simultaneously compacting said first and said second powder mixtures to form a green compact having two layers and sintering said green compact, wherein at least one of the chemical composition or the physical characteristics of at least one of said first and said second powder mixtures is adjusted so as to result in said valve seat face layer and said valve seat base layer having substantially the same density after compaction.

BACKGROUND OF THE INVENTION

The present invention relates to valve seat inserts for use in internalcombustion engines.

Valve seat inserts which are retained in place by an interference fit inthe cylinder head of an internal combustion engine are well known. Suchinserts have tended in the past to be made of a single material, eitherby a casting or by a powder metallurgy route followed by machining tosize.

More recently, two-layer valve seats made by powder metallurgytechniques have been made.

Two layer valve seat inserts comprise a seat face layer with which theseat of a popper valve usually makes contact, and a base or back-uplayer which is in contact with a receiving recess in the cylinder headfor example.

The functions fulfilled by each layer are distinct. Amongst otherthings, the seat face layer provides resistance to high temperature,hostile environments and repeated impact damage, whilst the base layerprovides long term creep resistance to ensure that the interference fitof the insert in its recess does not relax too much.

U.S. Pat. No. 4,485,147 describes a two layer valve seat insert havingcopper powder mixed with the powder material which forms the base layer.During sintering, the copper melts and infiltrates the valve seat insertface layer. This is said to save the cost of pressing and handlingseparate copper alloy infiltrating blanks.

EP-A-0130604 describes a two layer valve seat insert for a dieselengine, the insert having a base layer with improved creep and wearresistance over that of the seat face layer. The two layer seat insertwas produced by a double pressing operation. The valve seat inserts aremade by pre-compacting the base layer and subsequently compacting alayer of a seat face alloy onto the pre-compacted base layer.

In order to reduce the cost of a valve seat insert it is desirable toprovide the seat face layer in a material which is suitable for theservice conditions. However, it is desirable to provide the base layerin a material which is suitable for maintaining the integrity of theinterference fit in the cylinder head, but which material may begenerally less highly alloyed, and therefore less expensive, than theseat face layer.

Furthermore, it is also desirable for cost reasons, to reduce the numberof manufacturing steps involved in the production of a two layer valveseat insert. In this regard it is preferable to be able to compact bothpowder layers of the valve seat insert simultaneously. However,simultaneous compaction means that there is no individual control of thegreen densities of the two constituent layers.

According to a first aspect of the present invention, there is provideda method of making a two layer valve seat insert having a valve seatface layer and a base layer, the method comprising the steps ofpreparing two powder mixtures; a first powder mixture for forming thevalve seat face layer; a second powder mixture for forming the valveseat base layer; sequentially introducing a predetermined quantity ofeach of said first and said second powder mixtures into a powdercompacting die and having an interface therebetween substantiallyperpendicular to the axis of said die; simultaneously compacting saidfirst and said second powder mixtures to form a green compact having twolayers and sintering said green compact, wherein at least one of eitherthe chemical composition or the physical characteristics of at least oneof said first and said second powder mixtures is adjusted so as toresult in said valve seat face layer and said valve seat base layerhaving substantially the same density after compaction.

The term "substantially the same density" is herein defined as a densityvariation of not more than 3% between the two layers, and preferably notmore than 1.5%.

At least one of the first and second powder mixtures may have itschemical composition and/or physical characteristics such as powderparticle shape, size distribution and apparent density, for example,adjusted so as to achieve substantially the same density in each layer.

The term `mixture` is to be interpreted as meaning a mixture of at leasttwo dissimilar metal powders or a mixture comprising a single metalpowder but having one or more additions of, for example, lubricant wax,or an addition to promote machinability such as manganese sulphide orcarbon.

The density of each layer may be measured in either absolute terms as inMgm⁻³, or as a percentage of the theoretical density.

The properties of the subsequently sintered material are often stronglydependent on the initial green density. Therefore, it is desirable tomaintain the green density within a narrow band during cold compaction.The green density of each constituent layer is largely determined by therelative compressibility of the constituent powders. For a given powderblend the movement of the press ram (in a mechanical press for example)or the applied pressure (in a hydraulic press) and the depth of thepowder fill in the die controls the green density and the axialthickness in the pressing direction of the component. If the densitiesof the respective layers vary from each other, slight variations in therespective fill weights of each powder, as must necessarily occur, fromone pressing to another have a disproportionate effect on the size ofeach resulting valve seat insert produced. Thus, it is difficult tomaintain close dimensional control of the parts being produced. However,if the two constituent powders both exhibit the same or similarcompaction behaviour, as in the method of the present invention,monitoring and control of the size of the resulting green compacts aregreatly facilitated.

Generally, the powder mixture constituting the valve seat face layer ismore highly alloyed than that of the base layer. Thus, the valve seatface layer powder is generally consequently less compressible than thebase layer because of the high alloy content. Therefore, in oneembodiment of the present invention, the composition of the less highlyalloyed base layer powder is adjusted such that both the powders exhibitsimilar compressibility.

Adjustment of the base layer material may, for example, include themixing of different grades of iron powder. Such different grades maycomprise an atomised powder having a relatively high compressibility anda sponge iron powder having a relatively low compressibility, forexample. The relative proportions of each constituent powder may beadjusted so as to give an overall compressibility of the base layerpowder mixture substantially the same as that of the face layer powderto give a compact having substantially the same density in each of itstwo layers.

In addition to controlling the pressed densities of the two layers, itis also desirable to control the size change of each layer on sinteringso as to achieve a substantially equal size change in each layer.Substantially equal size change on sintering is desirable so as tominimise the amount of material which must be removed on post-sinteringmachining. Size control may be achieved by the addition of copper and/orcarbon powder in the form of graphite, for example, to the base layerand/or face layer powder mixtures. It has been found that additions ofgraphite powder to the base layer reduces expansion on sintering to alevel nearer that of the face layer. An addition in the range from about0.8 to 1.2 wt % has been found to be effective.

Sometimes, a post-sintering heat treatment may be employed. In this caseit is desirable to control the size change on heat treatment so as to besubstantially equal in both layers.

An addition of copper powder to the backing layer has been found toincrease expansion on sintering whilst a similar addition to the facelayer has been found to have a relatively lower effect on size changeupon sintering. Addition of copper powder is beneficial as it aids thesintering reaction as well as helping to control the size change onsintering.

In one embodiment of a two layer valve seat according to the presentinvention, the face layer may comprise a sintered ferrous-based alloyaccording to EP-B1-0 312 161 of common ownership herewith, the contentsof which are included herein by reference. Ferrous-based alloysaccording to claims 1 to 7 and made by the method described in claims 8to 14 of EP-B1-0 312 161 have been found to be particularly suitable forthe working faces of valve seat inserts.

Two layer valve seats according to the present invention may beinfiltrated with a copper-based alloy, preferably simultaneously during,or alternatively, subsequent to sintering. Furthermore, two layer valveseats according to the present invention may be infiltrated whether ornot the constituent layers have had copper additions made thereto in theinitial powder mixtures.

According to a second aspect of the present invention there is provideda two layer valve seat insert when made by the method of the firstaspect.

In order that the present invention may be more fully understood,examples will now be described by way of illustration only withreference to the accompanying drawings, of which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a graph of the effect of graphite additions on the sizechange of backing layer powders following sintering and heat treatment;and

FIG. 2 which shows a graph of the effect of admixed copper content onsize change following sintering and heat treatment.

PREFERRED EMBODIMENTS OF THE INVENTION EXAMPLE 1

A powder mixture for the seat face layer was prepared by mixing 49.5 wt% of a pre-alloyed steel powder of composition: 1%C; 4% Cr; 6% Mo; 3% V;6% W; Balance Fe with 49.5 wt % of an unalloyed atomised iron powder and0.5 wt % of graphite powder. An addition of 1 wt % of a lubricant waxwas also made.

A range of powder mixtures for the backing layer were made by mixing 70wt % of an atomised iron powder with 30 wt % of a sponge iron powder andfrom 0.6 wt % to 1.2 wt % of graphite powder. The addition of the spongeiron powder was made in order to reduce the compressibility of thebacking layer powder mixture to that of the face layer powder mixture.No further alloying additions were intentionally made. An addition of 1wt % of a lubricant wax was also made to each powder mixture.

A number of single layer pressings in the form of hollow cylindricalblanks were made from each of the powder mixtures, the pressing pressurebeing 770 MPa. Dimensions of the blanks were 6 mm axial thickness and6mm radial thickness. Blanks made from the face layer powder mixturewere coded "EF", whilst blanks made from the backing layer powdermixture were coded "CD". All the pressed blanks were infiltrated with acopper-based alloy during sintering which was carried out at about 1100°C. in an atmosphere of a hydrogen/nitrogen mixture.

Some two layer blanks were produced by the simultaneous compaction at770 MPa of two powder layers in a die. These blanks were also sinteredand infiltrated as in the blanks described above.

A post-sintering heat treatment was also effected comprising the stepsof cooling the sintered blanks to -120° C., followed by tempering at600° C. for 2 hours under a protective atmosphere.

Green density measurements were made on the pressed blanks as weredensity and size change measurements on the sintered articles and on thearticles following a post-sintering heat treatment.

FIG. 1 shows the effect of varying levels of carbon addition on the sizechange on sintering and subsequent heat treatment. As the carbon contentincreases, the expansion of the backing layer composition decreasestowards that of the face layer as shown by the horizontal line 10.

The green density of the seat face layer, EF, was 6.85 Mgm⁻³. Table 1below shows the green density of the backing layer compositions atvarying levels of carbon addition.

                  TABLE 1                                                         ______________________________________                                        C content of the Green Density,                                               backing layer alloy wt %                                                                       Mgm.sup.-3                                                   ______________________________________                                        0.6              6.88                                                         0.7              6.87                                                         0.8              6.86                                                         0.9              6.85                                                         1.0              6.86                                                         1.1              6.86                                                         1.2              6.85                                                         ______________________________________                                    

Table 1, therefore, shows that the compressibility of the backing layercompositions compares well with that of the face layer, EF, for a carbonrange from 0.6 to 1.2 wt %, whilst FIG. 1 shows that the expansion onsintering decreases with increasing carbon level. However,microstructural examination shows that at the lower levels of carbonaddition there is evidence of carbon depletion at the interface betweenthe two layers. This depletion is a result of the strong carbide-formingalloying elements in the seat face layer acting as a sink for thecarbon. However, at carbon levels above 1.2wt %, the microstructure ofthe two layer samples shows the backing layer to include somediscontinuous grain boundary carbides which is also undesirable. Thus,the desirable level of carbon in the base layer should be in the rangefrom 0.8 to 1.2 wt %. Significant carbon depletion in the backing layeris undesirable since adequate strength and hardness are required toensure that the valve seat insert is retained in the cylinder headduring operation of the engine.

EXAMPLE 2

Further examples of single layer and two layer pressings were made inthe non-infiltrated condition.

Powder mixtures for the face layer were as described above withreference to Example 1, but with the addition of 1 wt % manganesesulphide and copper powder in the range from 0 to 4 wt %.

Powder mixtures for backing layers having copper additions in the rangefrom 0 to 4 wt %, 0.5 wt % manganese sulphide and 1 wt % of carbon werealso prepared. The mixture of atomised and sponge iron powders were asdescribed with reference to Example 1.

Samples pressed from the seat face layer powders were coded "SF", whilstthose samples made from the backing layer powders were coded "BK".

Table 2 below shows the green densities in Mgm⁻³ of the face and backinglayers. In the table, the numeral following the layer code specifies thelevel of copper addition.

                  TABLE 2                                                         ______________________________________                                        Alloy       Cu wt %  Green Density Mgm.sup.-2                                 ______________________________________                                        SF-0        0        6.79                                                     SF-2        2        6.81                                                     SF-4        4        6.80                                                     BK-0        0        6.80                                                     BK-2        2        6.83                                                     BK-4        4        6.84                                                     ______________________________________                                    

Table 2 shows that the compressibility of the powder mixtures for thetwo layers were close for copper additions in the range from 0 to 4 wt %of copper. FIG. 2 shows that the size change on sintering of the facelayer is relatively insensitive to the addition of copper to the powdermixture. However, the size change on sintering of the backing layer ismuch more sensitive to the addition of copper. An addition of 2 wt % inthe backing layer causes a size change on sintering and subsequent heattreatment substantially the same as that of the face layer. Since theaddition of copper produces benefits in the strength of the sinteredmaterial as well as helping to control the size change on sintering, anaddition of between 2 and 4 wt % is desirable in non-infiltratedmaterial. This is fortuitous since the addition of copper in this rangehas long been known to act as a sintering aid for ferrous-basedmaterials.

I claim:
 1. A method of making a two layer valve seat insert having avalve seat face layer and a base layer, the method comprising the stepsof preparing two powder mixtures; a first powder mixture for forming thevalve seat face layer; a second powder mixture for forming the valveseat base layer; sequentially introducing a predetermined quantity ofeach of said first and said second powder mixtures into a powdercompacting die and having an interface therebetween substantiallyperpendicular to the axis of said die; simultaneously compacting saidfirst and said second powder mixtures to form a green compact having twolayers and sintering said green compact, characterised in that saidvalve seat face layer and said valve seat base layer have substantiallythe same green density after compaction and in that said two layers havesubstantially equal size change on sintering;said size change onsintering being controlled by a step selected from the group comprisingthe addition of up to 6 wt % copper to at least one of said powdermixtures;and;the addition of carbon powder in the range from 0.6 to 1.2wt % to the base layer powder mixture.
 2. A method according to claim 1characterised in that the density after compaction is determined inMgm⁻³.
 3. A method according to claim 1 characterised in that thedensity after compaction is determined as a percentage of thetheoretical full density.
 4. A method according to claim 1 characterisedin that at least one of the powder mixtures is a mixture of at least twodifferent constituent metal powders so as to achieve a desired compacteddensity.
 5. A method according to claim 4 characterised in that thepowder mixture constituting the valve seat face layer comprises a highlyalloyed ferrous-based powder and a relatively pure iron powder.
 6. Amethod according to claim 4 characterised in that the powder mixtureconstituting the valve seat base layer comprises a powder of arelatively high compressibility and a powder of a relatively lowcompressibility.
 7. A method according to claim 6 characterised in thatthe relatively high compressibility powder and the relatively lowcompressibility powder are both substantially pure iron powders.
 8. Amethod according to claim 6 characterised in that the relatively highcompressibility powder is an atomised iron powder and the relatively lowcompressibility powder is a sponge iron powder.
 9. A method according toclaim 1 from characterised in that the two layers have substantiallyequal size change on heat treatment after sintering.
 10. A methodaccording to claim 8 characterised in that the two layers havesubstantially equal size charge on heat treatment after sintering.
 11. Amethod according to claim 1 characterized in that the additions ofcopper lie in the range from 0 to 6 wt %.
 12. A method according toclaim 10 characterised in that said size change is at least partlycontrolled by additions of carbon powder to at least one of said powdermixtures.
 13. A method according to claim 12 characterised in that saidcarbon powder addition is made to the base layer powder mixture.
 14. Amethod according to claim 13 characterised in that the carbon powderaddition lies in the range from 0.8 to 1.2 wt %.
 15. A method accordingto claim 1 characterised by further including the step of infiltratingsaid two layer valve seat with a copper-based material.
 16. A two-layervalve seat insert characterised by being made by the method of any oneof claims 1 to 15.